Air-cooled heat exchangers, or air coolers, have been in use in the United States and elsewhere for many years. They are normally used where water for cooling is scarce, concern for water pollution is high, or the amount of heat to be removed is too low for water cooling to be economical. A discussion of air-cooled heat exchangers may be found in Perry's Chemical Engineers Handbook, 5th Ed., 1973, pages 11-23.
An air cooler normally consists of an array of finned tubes which contain the process stream to be cooled. A fan or blower is mounted above or below the tubes to force air across the tube bundle. The heated exhaust air is then dissipated to the atmosphere. Conventional air coolers are designed to handle large quantities of air with a small rise in air temperature. A variation of this is shown in U.S. Pat. No. 3,443,633 wherein the exhaust air is recirculated to prevent freezing during sub-zero operation.
Many attempts have been made to heat ambient combustion air prior to its use in a fired process heater or furnace. If the air is not preheated, then fuel must be used to heat the combustion air to the operating temperature of the furnace. Thus the higher temperature of the inlet combustion air the less fuel needed for the furnace.
Typically a steam coil or other heat source located at the furnace is placed in contact with the incoming air. Other methods include: a Ljungstrom Air Preheater, which exchanges heat from the flue gas exiting the furnace with the incoming air using a rotary device; U.S. Pat. No. 3,426,733 which discloses the use of a process stream to donate heat to the combustion air and then recover heat from the flue gases exiting the furnace; and U.S. Pat. No. 3,469,946 which discloses the use of a closed system containing a heat transfer liquid to exchange heat between the flue gas and the combustion air.
All of these methods suffer drawbacks ranging from reliability and corrosion problems to being non-economical. The present invention, however, employs hot air from process air coolers as the source of heated combustion air, requiring little in the way of extra mechanical equipment. There are no corrosion problems as associated with the use of flue gas. In addition, heated air that has in the past gone to waste, is now recovered and used to lower the fuel requirements of the furnace.